The present invention relates to a non-aqueous electrolyte secondary battery, particularly an improvement of a negative electrode used therefor.
There have been various vigorous studies on a non-aqueous electrolyte secondary battery including lithium or a lithium compound as a negative electrode, because it is to be expected to offer a high voltage as well as a high energy density.
To date, oxides and chalcogens of transition metals like LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2, MoS2 and the like are known positive electrode active materials for non-aqueous electrolyte secondary batteries. Those compounds have a layered or tunneled crystal structure that allows free intercalation and deintercalation of lithium ions. On the other hand, there are many previous studies on metallic lithium as the negative electrode active material. However, metallic lithium has a drawback that when used as the negative electrode active material, a deposition of lithium dendrites occurs on the surface of lithium during charging, which reduces charge/discharge efficiency or causes internal short-circuiting due to contact between formed lithium dendrites and the positive electrode. As one measure for solving such drawback, the use of a lithium alloy such as lithium-aluminum alloy which not only suppresses the growth of lithium dendrites but also can absorb therein and desorb therefrom lithium as the negative electrode has been under investigation. However, the use of such lithium alloy has a drawback that repeated charge/discharge operation causes pulverization of the alloy as the electrode, which in turn deteriorates the cycle life characteristics of a battery.
Therefore, there are proposals to suppress pulverization of the electrode by using a lithium-aluminum alloy including additional elements as electrode (e.g., Japanese Laid-Open Patent Publications Sho 62-119856 and Hei 4-109562). Under the circumstance, however, the improvement remains unsatisfactory. At present, lithium ion batteries have been put into practical use that include as the negative electrode a highly safe carbon material capable of reversibly absorbing and desorbing lithium and having exceptional cycle life characteristics although smaller in capacity than the above-mentioned negative electrode active materials. In an attempt to realize a higher capacity, various proposals have been made of an application of oxides to negative electrodes. For example, it is suggested in Japanese Laid-Open Patent Publications Hei 7-122274 and Hei 7-235293 that crystalline oxides such as SnO and SnO2 may serve as negative electrode active materials with higher capacities than the conventional oxide WO2. There is another proposal in Japanese Laid-Open Patent Publication Hei 7-288123 to use non-crystalline oxides such as SnSiO3 or SnSi1xe2x88x92xPxO3 for the negative electrode in order to improve the cycle life characteristics. But, the improvement is still unsatisfactory.
The primary object of the present invention is to provide a negative electrode for non-aqueous electrolyte secondary batteries having excellent charge/discharge cycle life characteristics.
Another object of the present invention is to provide a negative electrode affording a high electric capacity and an exceptional cycle life by absorbing lithium upon charging without growing lithium dendrites.
The present invention provides a non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte and a chargeable and dischargeable negative electrode, wherein the negative electrode comprises a compound represented by the formula (1)
DSnO3xe2x80x83xe2x80x83(1)
wherein D represents at least one alkaline earth metal.
In a preferred mode of the present invention, D is represented by the formula (2)
SrxBa1xe2x88x92xxe2x80x83xe2x80x83(2)
wherein 0.03xe2x89xa6xxe2x89xa60.5.
It is further preferable that the range of x is 0.1xe2x89xa6xxe2x89xa60.5 in the formula (2)
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.